Survey of Space Propulsion Concepts
Survey of Space Propulsion Concepts After a thorough examination of existing and proposed methods of space propulsion, it becomes apparent that they fit into a natural classification scheme. Current techniques can either be described as atmospheric, reaction, or field propulsion. Many future proposals can be categorized as ballistic, radiation, structural, and propellantless propulsion. Structural methods use various different systems for bearing loads and stresses. Propellantless concepts can be separated by the physical theory regarding their mechanism of operation. Atmospheric propulsion refers to methods that use the ambient partial pressure of free oxygen as the source of oxidizer for combustion or that rely on the principles of aerostatic and aerodynamic lift to remain airborne. Such vehicles are restricted to remain in Earth’s atmosphere, and usually must have pressurized cabins for passengers. Velocity is often limited by the speed of sound (Mach 1, about 340 m/s or 767 mph) and frictional heat. One of the definitions of the boundary between Earth and Space refers explicitly to these vehicles. The Karman line, at 100 km (about 62 miles, or about 328,000 feet), is the altitude at which the air has become too rarified to produce aerodynamic lift at a velocity less than orbital velocity. In practical terms, no airplane or glider has flown significantly higher than 120,000 feet. Reaction thrusters release propellant mass out the back, and this exchange of momentum results in the vehicle being propelled forward. This is a direct consequence of Newton’s Second Law. This is the well known F=ma that introductory physics students learn. When working with space propulsion it is instructive to consider the calculus of this equation by expressing it as F=dpdt=d(mv)dt. In the traditional Tsiolkovsky rocket equation, we have to consider the decreasing mass of the vehicle as fuel is consumed. If the propellant is a photon, we have no mass to work with, however, we do have momentum. If we have propellant with mass moving at near luminal velocities, we have to account for the relativistic effects on mass. These kinds of vehicles can all be considered a type of rocket because they carry the propellant mass on board. They can be called jets if they collect, accelerate, and then eject the ambient fluid or particle medium to develop thrust. The key parameter affecting these vehicles is the change in momentum, which is often assessed by comparing the specific impulse or thrust force. The other factor to consider is the intended flight velocity, referred to as the Ä''v''. Once the vehicle has entered a trajectory or orbit determined solely by the gravitational interaction, it can be said to be using field propulsion. It is using the properties of the gravitational field to propel itself. These vehicles use a preexisting exogenous force field to propel the spacecraft. Although it has not been done yet, spacecraft could also use electric and magnetic fields to determine their trajectory. Vehicles launching from a rotating body are also affected by the fields due to the inertial centripetal, centrifugal, and coriolis forces. A rotating mass also creates frame dragging and gravitomagnetic affects that distort the local spacetime. If it separates from a linearly accelerating body, it will experience additional inertial forces. The gravitational field gradient also produces differential forces on bodies, these are referred to as tidal forces. The orbital altitude at which these forces would cause a particular body to fall apart is known as the Roche limit. This is the process that is believed to have created planetary rings, like those of Saturn. In a system involving two large bodies, like the Sun-Earth system or Earth-Moon system, there are several regions where the gravitational forces cancel each other out, and an object can essentially come to “rest” here. These are known as Lagrange Points. Three of these points are co-orbital with the smaller body, and sometimes objects at these points are called Trojans. If we have two fluid bodies and they approach the Roche limit, a fluid “bridge” will form between the two bodies. Magnetic fields can form structures through their interaction with plasmas. Most charged particles will gyrate around field lines and proceed to flow to the poles. This effect has led to the formation of a “flux tube” between Jupiter and its moon Io. Some will get trapped into a stable toroid around the planet, for Earth, this has led to the formation of the Van Allen belts. All other particles get deflected. Finally, the Earth has an electric field, in the form of a vertical voltage gradient and the ionosphere. Other forms of propulsion have not been utilized yet, but there are many proposals that are quite different from current methods. A classic alternative is the ballistic method, popularized by Jules Verne. This uses a large piece of artillery in the form of a barrel, track, tunnel, ramp, cable, bow, or lever arm to propel payloads into ballistic, orbital, or escape trajectories. Some designs incorporate circular or spiral paths akin to particle accelerators. Newton indirectly suggested this idea with his gedanken cannon that would shoot a ball around the world and hit itself. Some of these devices work like a regular gun, by using the pressure created in a confined space through deflagration or detonation to launch a projectile. Gas pressure can also be created via thermal expansion, vaporization, sublimation, ionization, pumps, compressors, turbines, dump tanks, nozzles, pistons, accumulators, shockwaves, foils, vortices, and ram compression and combustion. We may have accidentally succeeded in doing this during one of the United States’ nuclear tests. Project Plumbbob involved detonating a device at the bottom of a vertical tunnel, upon which a round steel cover was placed. A series of high speed images of the steel cover taken during the test indicate that it flew off at a speed greater than escape velocity (11 km/s or 24,606 mph) imparting enough kinetic energy to reach space. Or it could have vaporized due to compression heating in the atmosphere. It has never been found. Other ballistic devices use rapidly changing electric and magnetic fields to push a projectile down a barrel. Some devices emulate medieval projectile weapons, such as slings, slingshots, bows, crossbows, and catapults. Other systems are based on flywheels, friction wheels, cables, chains, and hydraulics. The ballistic approach has critical limitations. One is the g-forces experienced by projectiles. This either requires very long barrels or heavy damping for manned launch vehicles. Another problem is the short trip through the atmosphere which will cause massive amounts of thermal stress. Fluid injection from the tip of the projectile can protect the ship, as can plasma shielding. Electromagnetic ballistic devices also require huge amounts of energy, produce lots of heat, and limit material choices. Radiative propulsion involves using ordinary light beams, lasers, alpha radiation, beta radiation, fission fragments, charged particle beams, neutral particle beams, antimatter beams, and magnetic particle beams to impart momentum or transfer energy to a traveling space craft. The radiation can also be emitted by natural objects, like the sun. The beam either reflects off of a mirror, plate, parabolic dish, sail, electric field, magnetic field, or ballistic launcher on the vehicle or transmits power to a receiver. It can also be used to ignite, ablate, or annihilate material at the rear of the craft. Traps, cooling lasers, tractor beams, and optical levitation are related processes that are in use today. The key advantage of such vehicles is that they do not have to carry the fuel. A major issue is the power required to create such beams and the fact that this method is less effective as distance increases. It also requires great accuracy and precision. Structural methods require large launch support architectures and structures. The most basic idea is that of the tall space tower. This would be a structure held together by compressive forces that allows vehicles to launch from above the densest parts of the atmosphere, significantly reducing the energy required to get to space. The other structure is the tensile counterpart to towers, tethers. Tensile launchers are cables used to pull and release vehicles into space. They can also be anchored to large bodies and work as elevators or cable cars. A third kind of structure combines compressive and tensile elements in the form of gas filled containers. They would be supported by the pressure and/or buoyancy of the filling gas. It could be as simple as a tethered balloon or aerostat, or columns of cylinders, spheres, and toroids. The highest balloons have gone to is about 53 km (33 miles or 174,000 feet), and balloon satellites have been deployed at 1600 km (990 miles or about 5.2 million feet). The final structural type is the dynamic or kinetic structure. This can be a cable or chain that is accelerated and launched upward or into a circle and is used to support an external structure. It can also be a stream of pellets that are launched up and deflected back down in a continuous loop. The propellantless drive is the holy grail of space propulsion. A successful version of this would revolutionize space travel. This would be a spaceship that does not need to carry its fuel or depend on an external source of propulsive energy. It would not be limited in range or size. Many theoretical designs have been proposed for such a space drive, each derived from the premises of different branches of physics. Some have looked at classical Newtonian mechanics and posited that negative or imaginary masses could exist and be used to develop novel means for space travel. Mach’s principle, which is the suggestion that there is an overarching reference frame in the universe upon which an object can exert a force and move itself forward, is an aspect of classical physics still up for debate but that would change the face of space travel. Relativity may permit the development of warp drives and wormholes, which would modify spacetime to move a vehicle through space. The study of cosmology implies that there is some kind of universal energy that induces the expansion of the universe. If we could somehow control this energy it could be used to propel a spacecraft. There are several quantum mechanical descriptions of the vacuum that imply it contains vast amounts of energy that could be tapped to propel a vehicle. The interaction between the electromagnetic and gravitational fields may provide a way to manipulate mass and spacetime in order to explore the universe, but requires a breakthrough in our understanding of physics. Some of the candidates for a grand unified theory also offer ways of achieving propellantless propulsion, but remain untestable with current techniques. Another mode of transport worth mentioning is the self-propelled levitating vacuum tube train. This would allow long distance travel at high velocities with minimal frictional losses. It could consist of elevated, terrestrial, subterranean, floating, or submarine tunnels. It can be enhanced through the use of gravity and pressure gradients, using brachistochrone curves and air pumps. It could also follow ellipsoidal, parabolic, and catenary paths and travel antipodally or along great circles. Active and passive electromagnetic or electrostatic levitation or even diamagnetic levitation would make it a contactless operation. The use of superconductors would make it more energy efficient. Friction could be further reduced through superlubricants and superfluids. Vehicles could be energized and propelled by traveling or changing electromagnetic fields or beams. It can also serve in tandem with ballistic and structural methods of space propulsion or mass driver relays used to transfer momentum to spacecraft.